Meconium aspiration syndrome (MAS), also referred to as “Neonatal aspiration of meconium”, occurs when infants inhale meconium into their lungs before or during delivery. Meconium is the first stool of an infant, composed of materials formed during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic fluid, bile, and water. Meconium is sterile, unlike later faeces, and has no odour.
Meconium is normally stored in the infant's intestines until after birth, but sometimes (often in response to fetal distress) it is expelled into the amniotic fluid prior to birth during late stage pregnancy or during labour. If the baby then inhales the contaminated fluid, respiratory problems may occur.
Meconium passage into the amniotic fluid occurs in about five to twenty percent of all births. It is more common in postmature births. Meconium aspiration syndrome develops in 5-10 percent of these cases. About a third of those infants who experience MAS require breathing assistance. The mortality rate for MAS resulting from severe parenchymal pulmonary disease and pulmonary hypertension is as high as 20%. Other complications include air block syndromes (eg, pneumothorax, pneumomediastinum, pneumopericardium) and pulmonary interstitial emphysema.
Frequently, fetal distress during labour causes intestinal contractions, as well as a relaxation of the anal sphincter, which allows meconium to contaminate the amniotic fluid. Amniotic fluid is normally clear, but becomes greenish if it is tinted with meconium. If the infant inhales this mixture before, during, or after birth, it may be sucked deep into the lungs. Two main problems occur if this happens:                The material may block the airways. Complete obstruction of the airways by meconium results in atelectasis. Partial or intermittent obstruction causes air trapping and hyperdistention of the alveoli, commonly termed the ball-valve effect. Hyperdistention of the alveoli occurs from airway expansion during inhalation and airway collapse around inspissated meconium in the airway, causing increased resistance during exhalation. The gas that is trapped, hyperinflating the lung, may rupture into the pleura (pneumothorax), mediastinum (pneumomediastinum), or pericardium (pneumopericardium).        The meconium-tainted fluid is irritant, and results in inflammation of the airways which can lead to chemical pneumonia. Enzymes, bile salts, and fats in meconium irritate the airways and parenchyma, causing a release of cytokines and resulting in a diffuse pneumonia that may begin within a few hours of aspiration. All of these pulmonary effects can produce gross ventilation-perfusion (V-Q) mismatch. To complicate matters further, many infants with meconium aspiration syndrome (MAS) develop pulmonary hypertension of the newborn (PPHN) as a result of failure of the pulmonary vessels to dilate thereby lowering pulmonary vascular resistance and pulmonary artery pressures. Finally, though meconium is sterile, its presence in the air passages can predispose the infant to pulmonary infection.        
The most obvious sign that meconium may have been aspirated is the greenish appearance of the amniotic fluid. The infant's skin may be stained green if the meconium was passed a considerable amount of time before birth. Rapid or laboured breathing, slow heartbeat, or low Apgar score are all signs of the syndrome. Inhalation can be confirmed by one or more tests such as using a stethoscope to listen for abnormal lung sounds, performing blood gas tests, and using chest X-rays to look for patchy or streaked areas on the lungs.
Although modern obstetric care has reduced the incidence of MAS, sudden unpredictable events occur during labour which stress the fetus to release meconium. Ensuring that the infant is born before 42 weeks of gestation may lessen the risk. Amnioinfusion is a method of thinning thick meconium that has passed into the amniotic fluid. In this procedure, a tube is inserted into the uterus through the vagina, and sterile fluid is pumped in to dilute thick meconium. Recent studies have not shown a benefit from amnioinfusion. Until recently it had been recommended that the throat and nose of the baby be suctioned by the obstetrician as soon as the head is delivered. However, new studies have shown that this is not useful and the revised Neonatal Resuscitation Guidelines published by the American Academy of Pediatrics no longer recommend it. When meconium staining of the amniotic fluid is present and the baby is born with depressed respiration, it is recommended that the pediatrician suction the mouth and nose and use a laryngoscope and suction catheter to suction meconium from below the vocal cords.
Other methods for treating MAS include surfactant instillation and surfactant lavage. WO 98/49191, for instance, describes a method for pulmonary lavage of a mammal suffering from MAS, said method comprising:    a) applying gas positive end-expiratory pressure (PEEP) with a ventilator into a lung section of said mammal at a pressure of from about 4 to 16 cm water;    b) instilling a lavage composition containing dilute surfactant in a pharmaceutically acceptable aqueous medium into said lung; and    c) removing pulmonary fluid from said lung using short intervals of tracheo-bronchial suction at a negative pressure of about 20 to 100 mm mercury.
Another method for treating MAS is described in WO 2004/075838 and comprises administering a complement inhibitor in the form of an antibody to a patient likely to develop or suffering from MAS. WO 2004/075858 teaches to administer the complement inhibitor to the lungs of the patient using inhalation or tracheal instillation.
The aforementioned methods suffer from the drawbacks that they are invasive and/or fail to effectively prevent or treat MAS.